In recent years, in an optical pickup apparatus, a wavelength of a laser light source which is employed as a light source for reproducing information recorded on an optical disc and for recording information on an optical disc, is becoming shorter. For example, a laser light source with wavelength λ1=0.39 to 0.42 μm, such as a blue-violet semiconductor laser and a blue-SHG laser in which wavelength of an infrared semiconductor laser is converted by utilizing a second harmonic wave, is reaching the stage of practical application. A use of these blue-violet laser light sources enables to record information of 15 to 20 GB on an optical disc with a diameter of 12 cm, when an objective lens with the numerical aperture (NA) which is the same as a DVD (Digital Versatile Disc) is used. When the NA is increased up to 0.85, information of 23 to 25 GB can be recorded onto an optical disc with a diameter of 12 cm. In this specification, an optical disc and an optical-magnetic disc which employ a blue-violet laser light source are referred as “high density optical discs” as their general name.
Hereupon, as for a high density optical disc using an objective lens with NA of 0.85, coma caused due to an inclination (skew) of the optical disc becomes larger. Therefore, some of the high density optical discs are designed so that its protective layer becomes thinner (which is 0.1 mm, while that of a DVD is 0.6 mm) than that of a DVD, to reduce the amount of coma due to the skew. On the other hand, it is hard to say that a value of an optical disc player/recorder (optical information recording and reproducing apparatus) as a product is not enough under just a condition that it can record and/or reproduce information for the above high density optical discs properly. Taking account of a fact that, at present, DVDs and CDs (Compact Discs) onto which various kinds of information have been recorded, are on the market, it is not enough that information can be recorded/reproduced only for the above high density optical disc properly. For example, realization of recording and/or reproducing information appropriately also for DVDs and CDs which are owned by users, leads to enhancement of its commercial value as an optical disc player/recorder for high density optical discs. From these backgrounds, an optical pickup apparatus installed in an optical disc player/recorder for high density optical discs is required to have a property to be capable of appropriately recording and/or reproducing information not only for the high density optical discs but also for DVDs and CDs, with maintaining compatibility.
As a method which is capable of recording and/or reproducing information adequately to anyone of high density optical discs and DVDs and further to CDs with maintaining compatibility, there can be considered a method to selectively switch an optical system for high density optical discs and an optical system for DVDs and CDs, corresponding to the recording density of an optical disc on which information will be recorded and/or reproduced. However, it is disadvantageous for the size-reduction and increases the cost, because it requires a plurality of optical systems.
Accordingly, in order to simplify the structure of an optical pickup apparatus and to intend the reduction of cost, it is preferable to make an optical system for high density optical discs and an optical system for DVDs and CDs into a common optical system, and to reduce the number of optical parts constructing the optical pickup apparatus as much as possible, even in the optical pickup apparatus with compatibility. Then, providing the common objective lens which is arranged with facing an optical disc, is most advantageous for the simplification of the construction and cost reduction of the optical pickup apparatus. Here, in order to obtain an objective lens which can be commonly used for plural kinds of optical discs for which different recording/reproducing wavelengths are used, it is required that an optical path difference providing structure having a wavelength dependency in spherical aberration, is formed on the objective optical system, to reduce spherical aberrations caused by a difference in wavelength and a difference in thickness of protective layers.
Patent Literature 1 discloses and optical element for use in an objective lens that includes two optical path difference providing structures and that is usable compatibly for high density optical discs and conventional DVDs and CDs.
Further, Patent Literature 2 discloses an optical element realizing compatibility by using a hologram which has a stair-shaped sectional form and includes step differences in its inner peripheral section for providing an optical path difference of about 1.25 times the wavelength for a light flux with 390 to 415 nm, and discloses an optical element which realizing compatibility by using a hologram which has a stair-shaped sectional form and includes step differences in its outer peripheral section for providing an optical path difference of about 0.25 times the wavelength for a light flux with 390 to 415
Patent Literature 3 discloses an objective lens that includes an optical path difference providing structure in which two basic structures are overlapped together, and that is usable compatibly for high density optical discs and conventional DVDs and CDs.